Screening centrifuges



Sept. 24, 1968 H. c. F. QYEN SCREENING CENTRIFUGES Filed Aug. l?, 1966 INVENTOR.

HAAKON C. F. OYEN TTOR N EY.

United States Patent Office 3,402,822 Patented Sept. 24, 1968 3,402,822 SCREENING CENTRIFUGES Haakon C. F. Oyen, Stamford, Conn., assignor to Dorr- Oliver Incorporated, Stamford, Conn., a corporation of Delaware Filed Au". 17, 1966, Ser. No. 572,981 Claims. (Cl. 210-374) The present invention relates to screening Centrifuges of the type having a conical screen and a helical conveyor therein to move solids along the screen. In such Centrifuges, it is common to introduce the feed material at the narrow end of the screen. Centrifugal force urges the feed material radially outwardly against the screen where there occurs a separation of one fraction of the feed material, for example a liquid, which passes through the screen, from a second fraction, for example solids, which is retained on the screen.

Because of the conicity of the screen, the component of the radial centrifugal force pa-rallel to the screen urges the solids toward the wide discharge end of the screen. The conveyor of the centrifuge has helical vanes and is positioned within the screen with a predetermined clearance therebetween. As is conventional, the conveyor is rotated with the screen but at a speed ydifferential whereby the helical vanes effect an apparent traverse from the narrow to the wide end of the screen and thus control the axial transfer of the solids along the screen.

This control of the solids fraction by the vaned conveyor or helix is effected under certain conditions by pushing the solids down the screen and under other conditions by keeping the solids from sliding down the screen too fast. Which of these two control techniques exists in a given centrifuge application is dependent on a great many factors and, at the present `state of the art, is generally unpredictable. It should be noted that observation of the action within a centrifuge during operation is not practically possible. However, from examination of wear patterns on the helix vanes, there is an indication that in certain applications, the helix operates to retard the solids near the feed end of the rotor and to push the solids along near the discharge end.

The present invention results from a problem occurring in the use of such prior art screening Centrifuges. The problem phenomenon occurred when the centrifuge feed rate was increased above the point which was presumed by the art to be maximum machine capacity. Such increased feed rates were met with marked vibration and with a rapidly increasing torque between the differentially rotating screen and helix.

As is conventional in such machines, the differential transmission which -drives the screen and the conveyor is protected fr-om torque overloads by a cut-out device which permits screen and helix to rotate at the same speed while stopping the feed to the centrifuge. Consequently the result of attempting a feed rate increase was to trip the transmission cut-out. Yet, from a theoretical viewpoint, the machine was nowhere near a maximum capacity as determined by the available space for the solids between the screen and the helix.

Numerous unsuccessful attempts to determine the causes of the vibration and the torque overloads yand to eliminate same were tried. Although slower operating speeds alleviated the vibrational problems, the torque overload condition persisted. A proposed solution was to increase the clearance between the helix vanes and the screen which is normally in the range of .010" to .100". However, even with increased clearances the torque overloads still occurred when the feed rates were increased above the conventional maximum.

Finally it was theorized that at a certain critical value of the feed rate, the upward reaction on the helix from the solids being pushed downwardly equaled or exceeded the downward reaction from the solids being retarded plus the weight of the helix. Consequently, the helix was lifting within the screen as permitted by the customary axial play in the ball bearings which rotatably support the helix. This lifting of the helix with respect to the screen effected a wedging of the helix toward the screen. This wedging action was squeezing a layer of solids which remains between the opposite surfaces of the vane and the screen after passage of the leading edge of the vane. This compression of the solids layer was causing greatly increased drag and resulting torque on the differential transmission.

Additionaly, at the above-mentioned critical feed rate, the helix was floating and was therefore free to oscillate vertically within the limits of the axial bearing play. Coincident to this vertical freedom, the helix ball-bearings, which when unloaded axially have a conventional degree of radial play, were allowing radial helix motion. Consequently, at the critical feed rate, the prior art bearing arrangement permitted vertical oscillation and radial wobbling of the helix. These factors combined to create the severe vibration experienced.

To check these theories, a scribing point was mounted on a screen-associated component of the prior art centrifuge with a .005" vertical clearance from a horizontal surface on the helix. The centrifuge was run up to the vibration and overload Value of feed rate. Subsequent examination showed a score in the helix and a broken scribe point, thus indicating the helix had lifted at least .005".

With a view to the newly discovered causes of the vibrational and overload problems, the present invention provides a solution to these problems by restraining t-he helix axially with respect to the screen. In the illustrated embodiment, this restraint is provided by a zero-end-play bearing arrangement as fully described below. Centrifuges with this unique bearing arrangement have been tested and found to be free of the vibrational problem. Further, the improved Centrifuges can handle, at the maximum permissible torque for the differential drive, a feed rate approximately 240% greater than the same Centrifuges having the prior art bearing arrangement.

Accordingly it is a primary object of the present invention to provide screening Centrifuges having greatly improved capacity and vibrational characteristics as compared to prior art devices.

Further objects of the invention include:

(l) The provision of screening-centrifuge rotors having zero axial end-play between the helix and the screen;

(2) The provision of an improved screening-centrifuge bearing arrangement which prevents axial movement of the helix with respect to the screen;

(3) And the provision of screening Centrifuges having a preloaded bearin-g between the drive shafts for the screen and the helix to prevent helix-shift-induced drag between the screen and the helix.

These and other objects of the present invention will appear from the following description and appended claims when read in conjunction with the accompanying drawings wherein:

FIGURE 1 is a vertical section of the screening centrifuge of the present invention incorporating a zero-endplay bearing between the dri-ve shafts for the centrifuge screen and helix to prevent axial movement therebetween.

FIGURE 2 is a detail section showin-g the clearance between the opposite faces of the helix vanes and the screen of FIGURE 1 and shows the layer of solids which occupies this clearance during operation.

FIGURE 3 is an enlarged, fragmentary, vertical section showing in exaggerated manner the raceway offsets of the Considering the detail structure of the illustrated rotorl of FIGURE 1, the screening centrifuge of the present invention includes a rotor 11 having an inlet opening 12 for feeding process material to the rotor as indicated by arrows 14. The feed material passes to an annular conical space 16 in the rotor between a foraminous conical screen 19 (FIGURE 2), which is carried on the interior surface of a rapidly rotating perforated cage or screen support 18, and a rotating conveyor or helix 20. The rotation of cage 18 and screen 19 urges the process material against the screen. The screen retains one portion of the feed material, for example solids, while another portion of the feed material, for example a liquid, is centrifugally forced through the screen as indicated by arrows 22. The liquid is collected in one or more chambers of the centrifuge housing (not shown) in conventional manner. The material 23 (FIGURE 2) retained on screen 19` moves downwardly under control of vanes 40 on the helix. The pictorial representation of helical vanes 40 has been simplified in FIGURE 1 to show the screen-to-helix clearance throughout the axial length of the rotor whereas in FIG- URE 2 the true sectional appearance of a helical vane is shown. The helix rotates in the same direction as screen 19 but at a speed differential whereby an axial conveying action is effected. The solids are finally discharged from rotor 11 through openings 42 as indicated by arrows 44 and are collected in a solids chamber of the centrifuge housing (not shown) in conventional manner.

A drive head assembly 58 is provided to rotatably support and drive the screen and the helix of the rotor. Drive head assembly 58 includes a fixed, double-coneshaped, support housing 60 havin-g bearing assemblies 62 and 64. An annular outer shaft assembly 66, which drives cage 18, is mounted to rotate within these bearing assemblies. A coaxial inner shaft assembly 68, which drives helix 20, is mounted by bearing `assemblies 70 and 72 to rotate within outer shaft assembly 66.

The lower ends of shaft assemblies 66 and 68 are driven at differential speeds by a suitable drive arrangement such as transmission 80 which is completely described in U.S. Patent 3,332,300, issued July .25, 1967. The transmission is powered by a belt pulley 82 which is in turn driven by a belt or belts 84 which extend from a power source such as a motor (not shown).

Now referring to the above-mentioned major components of the centrifuge in greater detail, rotor 11 includes annular rings 90 and '92 suitably secured to the upper open end of cage 18 to form feed inlet opening 12. Cage 18 is formed by a conical member 94 which has a plurality of perforations or slots 96 therein for passage of effluent to the fluid chambers of the centrifuge housing (not shown) which surrounds the rotor. The cage is secured by screws 98 to a spider ring 100 containing the solids apertures 42. The ring is in turn secured by screws 102 to a bell-like hub 104, and the hub is non-rotatably connected to a hollow shaft 106 of outer shaft assembly 66 by any suitable means such as cap screws (not shown).

Helix 20 includes a lower conical portion 110 secured by a key 114 to rotate with a shaft 112 of inner shaft assembly 68. An upper conical portion 116 of the helix is connected to lower portion 110 by cap screws 118. In the particular embodiment illustrated, helix 20 of the screening centrifuge is provided with two series of passages 122, 124 and 126, 128, 130, 132 to supply a wash fluid or fluids from concentric wash fluid feed pipes 134 and 136 to the material being processed in the rotor.

Outer shaft assembly 66 further includes a bearing spacer 140 between the inner races of outer shaft bearing assemblies 62 and 64; a lock nut assembly 148 to clamp the inner races of bearing assembly 62, bearing spacer 140, and the inner races of bearing assembly 64 all in place against a shoulder 149 of outer rotor shaft 106;

and a clamping rring` 146 to retain the outer-races of inner shaft bearing assembly 70 against an outer shaft shoulder 147. Inner shaft assembly 68 is provided with a sealing arrangement 142 between cage hub 104 and inner shaft 112 and a lock nut assembly 144 to clamp the inner races of bearing assembly 70 upon a shoulder 145 of inner shaft 112. Support housing 60 includes a seal assembly 150 to provide a seal between the stationary housing and the rotating hub 104. Housing 60 is further provided with a retainer 152 to maintain the 'outer races of bearing assembly 64 in position against shoulder 153 of the housing.

Bearing arrangements Considering the bearing arrangements and their functions in greater detail, it is customary in conveyor-type screening :centrifuges to support each of the rotary components in two spaced-apart anti-friction bearing assemblies. One of these bearing assemblies serves to position the component axially, within the limits of the bearing play, and the other bearing assembly is arranged to permit axial floating to compensate for manufacturing tolerances and for differential thermal expansion of the housing and/or shafts. Thus in 4the present invention outer shaft 106 is axially positioned with respect to the housing by lower bearing assembly 64, the inner and outer races of lwhich are rigidly mounted on the shaft and housing, respectively. However, the outer shaft upper bearing assembly 62 is mounted with its outer races floating in the housing for the above-mentioned purposes. Similarly, inner shaft 112 is positioned axially with respect to outer shaft 106 by upper bearing assembly 70, the inner and outer races of which are rigidly mounted on inner shaft 112 and in outer shaft 106, respectively. But, the lower end of inner shaft 112 is free to float axially with respect to the outer shaft in roller bearing assembly 72.

The bearings supporting the inner and the outer shafts conventionally have quite different characteristics for the following reasons. Because of the differential drive arrangement, the relative speed between the inner shaft and the outer shaft is much less than the relative speed between the outer shaft and the housing. For example, in one specific centrifuge the outer shaft rotates at 1100 r.p.m. while the differential speed between shafts is only 44 r.p.m. The differential speed between shafts is generally in the range of 25 to 120 r.p.m. in screening centrifuges. Accordingly, the outer shaft bearings Aassemblies 62 and 64 operate at much higher speeds than inner shaft bearing assemblies 70 and 72. Consequently, it is conventional to provide outer shaft bearings which are heavier duty than the inner shaft bearings and which in many cases are double-row, zero-radial-play bearings to alleviate vibrational problems at these relatively high rotational speeds. In contrast, the prior lart inner shaft bear-ings commonly have been single row bearing arrangements.

It should be noted that single-row rolling-element bearings inherently have a degree of end play. This end play can be eliminated only if special provisions or procedures are utilized in a double-row bearing arrangement. Therefore, in order to prevent axial helix movement within the screen, a zero-end-play, double-row bearing is provided in the present invention as the upper bearing between the shafts. By positioning the zero-end-play bearing between the upper or helix/ screen proximate ends of the shafts, helix shift due to differential thermal expansion of the shafts is avoided. f-

In the invention as illustrated, a double-row, zero-endplay bearing assembly 70, in which the rolling elements are balls, is utilized between the shafts. A shim or shims are inserted during installation between the respective inner races '182 and 184. Outer races 186 and 188 are forced together during assembly by clamping ring 146 so as to preload the bearing. This forcible clamping creates an offset between the centerlines and 192 of the raceways of races 182 and 186. A similarv but oppositely directed offset is created between centerlines 194 and 196 of the raceways of races 184 and 188, respectively. The effect of these raceway offsets is depicted in exaggerated manner in FIGURE 3 by the contact relationships between the balls and the raceways and by the illustrated force arrows indicating the direction of preload forces between the races and the balls.

As exemplary of this technique for a specific centrifuge application, shims were selected to create an axial preload in the range of 90 to 130 pounds. It should be noted that alternatively a similar although inverse preload relationship could be effected by a shim between the outer races with the inner races being clamped together. Other zero-end-play bearing techniques are visualized for incorporation in the present invention. For example, instead of installing shims between either the inner or the outer bearing races, a portion of one or both of the inner or outer races can be removed as by grinding to achieve analogous raceway offsets and preloads. Other conventional zero-end-play bearing arrangements can also be used in the present invention.

As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is illustrative and not restrictive. The scope of the invention is defined by the appended claims rather than by the description preceding them, and all embodiments which fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by those claims.

I claim:

1. In a screening centrifuge having a conical screening means, a coaxial conical conveying means therein, and respective coaxial shaft means connected at one end to said screening means and to said conveying means to rotatably drive same, said conveying means including helical vanes and rotating at a speed differential with respect to said screening means to move material along said screening means away from the apex endthereof; the improvement comprising Zero-end-play bearing means between the respective coaxial shaft means to prevent said conveying means from moving axially toward said screening means during operation, said bearing means being a double-row rolling-element bearing system arranged with an axial preload on one of the rows of rolling-elements reacted against a preload on the other row of rolling ele ments.

2. An apparatus as defined in claim 1, wherein said bearing system is a double-row ball bearing arrangement with a preload in the range of 90 to 130 pounds axially.

3. An apparatus as defined in claim 1, said zero-endplay bearing means being located adjacent the ends of the respective coaxial shaft means which are connected to said screening means and to said conveying means.

4. A screening centrifuge rotor comprising:

(a) a conical screen-supporting perforated cage;

(b) a conical screen fitted upon the internal surface of said cage to screen material being treated in the rotor;

(c) a conical conveyor having helical -vanes positioned coaxially within said screen with a predetermined clearance between said vanes and said screen;

(d) respective inner and outer coaxial drive shafts connected at one end to said cage and to said conveyor;

(e) `differential drive means connected to the other end of said shafts to drive the shafts in the same direction at different speeds, the speed differential being such as to effect an apparent motion of said helical vanes from the narrow end of said conical screen toward the wide end thereof;

(f) and respective anti-friction bearings between said shafts at the drive proximate ends thereof and at the screen/helix proximate ends thereof, said screen/ helix proximate bearing having a double row of bearing balls, each row ha-ving a respective inner and outer raceway, the inner and outer raceways for each ball row being rigidly positioned on the respective shafts with an axial offset between the raceways to axially preload each row of balls, the raceway offset for one row of balls being opposite to that of the other row whereby the axial preloads on the respective rows of balls are reacted against each other thereby preventing shifting of said helix with respect to said screen during operation.

5. A centrifuge rotor as defined in claim 4 wherein each said row of balls has separate inner and outer races and wherein a shim means is installed between the respective races on one of said shafts with the respective races on the other shaft being clamped together to establish said opposite axial raceway offsets.

References Cited UNITED STATES PATENTS 2,043,661 6/1936 Howe 210-374 X 2,370,353 2/1945 Howe 210374 3,071,228 1/1963 Zeidler et al 308-189 X 3,332,300 7/1967 Nyrop 210-374 X REUBEN FRIEDMAN, Primary Examiner.

I. DE CESARE, Assistant Examiner. 

1. IN A SCREENING CENTRIFUGE HAVING A CONICAL SCREENING MEANS, A COAXIAL CONICAL CONVEYING MEANS THEREIN, AND RESPECTIVE COAXIAL SHAFT MEANS CONNECTED AT ONE END TO SAID SCREENING MEANS AND TO SAID CONVEYING MEANS TO ROTATABLY DRIVE SAME, AND CONVEYING MEANS INCLUDING HELICAL VANES AND ROTATING AT A SPEED DIFFERENTIAL WITH RESPECT TO SAID SCREENING MEANS TO MATERIAL ALONG SAID SCREENING MEANS AWAY FROM THE APEX END THEREOF; THE IMPROVEMENT COMPRISING ZERO-END-PLAY BEARING MEANS BETWEEN THE RESPECTIVE COAXIAL SHAFT MEANS TO PREVENT SAID CONVEYING MEANS FROM MOVING AXIALLY TOWARD SAID SCREENING MEANS DURING OPERATION, SAID BEARING MEANS BEING A DOUBLE-ROW ROLLING-ELEMENT BEARING SYSTEM ARRANGED WITH 